The invention relates to multipass inkjet printing.
A typical inkjet printer reproduces an image by ejecting small drops of ink from a printhead containing nozzles, where the ink drops land on a receiver medium (typically paper) to form ink dots. A typical inkjet printer reproduces a color image by using a set of color inks, usually cyan, magenta, yellow, and black. It is well known in the field of inkjet printing that if ink drops placed at neighboring locations on the page are printed at the same time, then the ink drops tend to flow together on the surface of the page before they soak into the page. This can give the reproduced image an undesirable grainy or noisy appearance often referred to as xe2x80x9ccoalescencexe2x80x9d. It is known that the amount of coalescence present in the printed image is related to the amount of time that elapses between printing adjacent dots. As the time delay between printing adjacent dots increases, the amount of coalescence decreases, thereby improving the image quality. There are many techniques present in the prior art that describe methods of increasing the time delay between printing adjacent dots using methods referred to as xe2x80x9cinterlacingxe2x80x9d, xe2x80x9cprint maskingxe2x80x9d, or xe2x80x9cmultipass printingxe2x80x9d. There are also techniques present in the prior art for reducing one-dimensional periodic artifacts or xe2x80x9cbands.xe2x80x9d This is achieved by advancing the paper by an increment less than the printhead width, so that successive passes or swaths of the printhead overlap. The techniques of print masking and swath overlapping are typically combined. See, for example, U.S. Pat. Nos. 4,967,203 and 5,992,962. The term xe2x80x9cprint maskingxe2x80x9d generically means printing subsets of the image pixels in multiple passes of the printhead relative to a receiver medium.
Another attribute of modern inkjet printers is that they typically possess the ability to vary (over some range) the amount of each ink that is deposited at a given location on the page. Inkjet printers with this capability are referred to as xe2x80x9cmultitonexe2x80x9d inkjet printers because they can produce multiple density tones at each location on the page. Some multitone inkjet printers achieve this by varying the volume of the ink drop produced by the nozzle by changing the electrical signals sent to the nozzle or by varying the diameter of the nozzle. See, for example, U.S. Pat. No. 4,746,935. Other multitone inkjet printers produce a variable number of smaller, fixed size droplets that are ejected by the nozzle, all of which are intended to merge together and land at the same location on the page. See, for example, U.S. Pat. No. 5,416,612. These techniques allow the printer to vary the size or optical density of a given ink dot, which produces a range of density levels at each location, thereby improving the image quality.
Another common way for a non-multitone inkjet printer to achieve multiple density levels is to print a small amount of ink at a given location on several different passes of the printhead over that location. This results in the ability to produce a greater number of density levels than the nozzle can fundamentally eject, due to the build up of ink at the given location over several passes. See, for example, U.S. Pat. No. 5,923,349.
In all of the above mentioned inkjet printers, the designer of the printer is additionally faced with the task of splitting the image data into multiple memory buffers corresponding to the multiple passes of the printhead.
There is need for improvement over the prior art in the area of multipass printing to support both multitone and non-multitone ink jet printers which eject multiple drops at a given location on several different passes. There is also a need to combine this capability with print masking. This is necessary to maximize the image quality of the ink jet printer by minimizing coalescence and preventing the excessive use of ink.
It is an object of the present invention to provide for multipass inkjet printing which produces high quality images.
This object is achieved by a method of multipass inkjet printing of images in response to digital images having code values, comprising the steps of:
a) providing an inkjet printer capable of printing at least two ink drops at each printing location during different passes, including a printhead which can select at least two different ink drop sizes; and
b) selecting a number of passes for producing an image and providing a print mask having mask values which, in each pass, sequentially corresponds to predetermined pixels and, in response to the mask values and the code values of such pixels, operates the inkjet printer to select the ink drop size for each pixel in each pass so that the inkjet printer produces an image.
The present invention determines the ink volume or drop size to be printed at a given location on a receiver medium using an inkjet printer which is capable of ejecting ink on multiple print passes at each given location. This enables the creation of more color levels without requiring excessive amounts of ink.
It is an advantage of the present invention that it may be applied to both multitone and non-multitone ink jet printers.
It is another advantage of the present invention that print masking is achieved in order to minimize coalescence.
It is another advantage of the present invention that banding artifacts may be reduced by swath overlapping.
It is another advantage of the present invention that a wide variety of ink and media combinations can be accommodated.